Caption: Macrophage engulfing E. coli. Coloured scanning electron micrograph (SEM) of a white blood cell (macrophage, yellow) engulfing Escherichia coli bacteria (pink rods). This process is called phagocytosis. Macrophages are cells of the body's immune system. They phagocytose and destroy pathogens, dead cells and cellular debris. E. coli bacteria are found in the lower intestines of most mammals. They are usually harmless, but some strains can cause food poisoning, septicaemia or meningitis. Magnification: x14000 when printed at 10 centimetres wide.